Separation and concentration of a liquid mixture by a membrane separation method, which does not involve phase change, is energy-saving compared with conventional separation techniques such as distillation, and is widely used in various fields including food fields such as concentration of fruit juice, and separation of beer yeast, and recovery of organic matters from industrial waste water because it does not involve state change of a substance. The water treatment using a membrane has become established as an essential process for supporting the leading-edge technology.
Such a water treatment using a membrane is conducted by using a membrane module in which a membrane element consisting of an assembly of membranes as one constituent is changed in a pressure vessel. In particular, a hollow-fiber membrane element is advantageous in that high water permeation flow rate is achieved as a whole and the volume efficiency is very high because a large membrane area can be ensured per volume of the membrane module although the water permeation flow rate per unit membrane area is not high compared with a spiral membrane element, and is excellent in compactness. In addition, when both a high concentration aqueous solution and fresh water are fed into the module and they are brought into contact with each other via a semipermeable membrane, the concentration polarization on the membrane surface can be controlled low.
In the case of a hollow-fiber reverse osmosis membrane, a both open-ended membrane is employed from the aspect of efficiency (see Patent Literatures 1 and 2). As shown in the illustration of FIG. 1, for example, the current of the membrane-permeated water in that case flows into the hollow-fiber membrane (inside the bore) from outside the same, and flows out through the opening part of each end. The flow length traveled by the membrane-permeated water in the bore is about half the total length of the hollow-fiber membrane as is apparent from FIG. 1. In this case, since seawater flows outside the hollow-fiber membrane, and the outside of the hollow-fiber membrane is pressurized, the current in the direction of pressurizing to adhere contaminants against the membrane surface occurs, and contamination components in the seawater are captured and deposited between neighboring hollow-fiber membranes to pollute the membrane element. This tends to adversely influence on the performance.
Also in the case of a hollow-fiber forward osmosis membrane, a both open-ended membrane is employed (see Patent Literature 3). The current of the membrane-permeated water in this case flows from inside (inside the bore) to outside the hollow-fiber membrane, for example, as shown in the illustration of FIG. 2. For example, in the case where a draw solution (DS; seawater) of high osmotic pressure flows outside the hollow-fiber membrane, and a feed liquid (FS; fresh water) of low osmotic pressure flows the bore of the hollow-fiber membrane, the membrane-permeated water flows from inside to outside the hollow-fiber membrane. In this case, the fresh water which is the source of the membrane-permeated water flows the bore of the hollow-fiber membrane as is apparent from FIG. 2, and flows from one end to the other end of the hollow-fiber membrane, and the flow length thereof is equivalent to the total length of the hollow-fiber membrane. Therefore, pressure loss in flow in the bore in the case of the forward osmosis membrane (FO membrane) is significantly larger than that in the case of the reverse osmosis membrane (RO membrane).
In the case of an RO membrane, in order to prevent membrane contamination by the membrane-permeated water of the hollow-fiber membrane, fouling resistance is improved by arranging the hollow-fiber membranes constituting the membrane element in an intersecting manner, for example, in Patent Literature 1. Specifically, by forming an intersecting part of hollow-fiber membranes, a gap between hollow-fiber membranes is produced, and thus occurrence of a channeling flow or concentration polarization is prevented, and muddy components of seawater are difficult to be pooled on the outside surface of hollow-fiber membranes. In this case, a larger number of winds per element length of the hollow-fiber membranes arranged in an intersecting manner is preferred, and as a result, the number of intersecting parts of hollow-fiber membranes increases, and the fouling resistance is improved. In Patent Literature 1, as is apparent from a drawing, an RO membrane having a number of winds of two is disclosed. Also in Patent Literature 2, an RO membrane having a number of winds of two is specifically disclosed.
When the structure of an intersecting arrangement with a number of winds of two is employed in the case of an FO membrane, the pressure loss of FS flowing in the bore is large, and water permeation performance possessed by the hollow-fiber membrane is not satisfactorily exerted. This is because the pressure loss in flow of FS flowing in the bore of the hollow-fiber membrane is large, and in particular, in the case of forward osmosis, the influence on the pressure loss in flow by a larger number of winds is about twice the case of the RO membrane. Therefore, the intersecting arrangement of hollow-fiber membranes employed in the RO membrane cannot be directly employed in the FO membrane.
As described above, it is the current state of art that no useful means for improving the fouling resistance of hollow-fiber membranes for FO has been found, and the measure achieving the fouling resistance while ensuring sufficient water permeation performance has not been realized even if the intersecting arrangement of hollow-fiber membranes for RO is taken into account.